When applying a coupon at Checkout, an error says Discount Coupon has expired or is not valid.

All our coupon or voucher codes have an expiry date.  Also coupons apply to specific products only.

Please review the coupon code and make sure it is entered correctly for the RIGHT product.  If you entered the coupon code correctly, you will need to get another coupon that would apply for the product and that is current.

That is the only way - to get a current coupon for the product if you want to get a discount.